1. Field of the Invention
The present invention relates to cardiac pacemakers and more particularly to pacemakers which combine sensor based, rate determining algorithms with atrial synchronized pacing.
2. Description of the Prior Art
The first pacemakers operated asynchronously with respect to the patient's underlying heartbeat. These VOO mode pacemakers are typified by U.S. Pat. No. 3,057,356 to Greatbatch. Although the VOO pacing modality was sufficient to sustain life, the occasional competition between the pacing stimulus and the natural heartbeat of the patient was considered undesirable.
In response, the VVI mode, or ventricular inhibited pacing modality, as typified by U.S. Pat. No. 3,345,990 to Berkovits, was introduced. This modality eliminates competitive pacing with naturally occurring ventricular rhythms. This type of pacemaker provides a sense amplifier to detect the ventricular beat of the patient's heart. The detection of a ventricular beat inhibits the generation of a ventricular stimulus. The detected beat also resynchronizes the pacemaker by recycling or restarting the V-V escape interval timer. This synchronized behavior causes the VVI pacer to fill in missed beats on demand.
The atrial synchronized pacing modalities, such as the VAT mode typified by U.S. Pat. No. 3,254,596 to Keller, provide an atrial sense amplifier for detecting atrial depolarizations of the patient's heart. In these pacers, the atrial event initiates an A-V delay timer which will provoke a ventricular pace event if a naturally conducted ventricular beat does not follow the spontaneous atrial beat. These pacemakers provide the hemodynamic benefits of A-V synchrony along with the ability to follow or track the naturally occurring atrial rate of the patient.
The benefits of A-V synchrony are also provided by the DVI mode pacemaker which operates on the A-V sequential principle. This device is typified by U.S. Pat. No. 3,595,242 to Berkovits. This form of pacemaker provides stimulation in both the atrium and the ventricle while providing for ventricular sensing and synchronization. In this form of pacemaker, a ventricular sense event initiates a V-A timer as well as a V-V timer. At the expiration of the V-A interval, the pacemaker provides a stimulus to the atrium. In a similar fashion at the expiration of the V-V time interval, the pacemaker will provide a ventricular stimulus to the patient's heart. Each of these events will occur unless during the concurrent V-A and V-V time interval there is a sensed ventricular beat which will recycle or restart both timers. Therefore this pacer does not compete with ventricular rhythms but may compete in the atrium. In the DDI mode pacer, typified by U.S. Pat. No. 3,747,604 to Berkovits, there is also included an atrial sense amplifier for inhibiting the generation of the atrial pacing stimulus at the conclusion of the V-A interval, if a natural atrial depolarization occurs within that V-A time interval. This pacing modality also incorporates a V-V timer and provides both ventricular and atrial pacing to restore A-V synchrony while preventing both atrial and ventricular competition.
The DDD pacemaker is typified by U.S. Pat. No. 4,312,355 to Funke and shares many of the characteristics of the DVI and DDI pacers but in addition permits the synchronization of the pacemaker upon the atrial rate. The DDD pacer provides both an A-V timer and a V-A timer. In operation, the V-A timer is started upon the occurrence of a ventricular event, either sense or pace, and the A-V timer is started upon either atrial sense or pace. At the conclusion of the respective time intervals, atrial and ventricular stimuli are delivered to the heart if required.
All of the atrial synchronized pacemakers discussed above track the patient's underlying atrial rate when it is appropriate. However, each of these devices must provide mechanisms to prevent tracking atrial rates at high rates which are pathologic. Likewise, these pacemakers must provide a backup or standby pacing rate in the absence of detected atrial activity.
The upper rate characteristic of atrial synchronized pacemakers has been the subject of much study and debate. See, for example, "Dual Chamber Pacemakers: Upper Rate Behavior", PACE, Vol. 8, March/April 1985.
The upper rate characteristic of the pacer of the present invention is related to the autodecremental upper rate behavior set forth in "Improved DDD Pacing with a New Rate-Limiting Algorithm" by Barouh V. Berkovits published in the Proceedings of the VIIIth World Symposium on Cardiac Pacing and Electrophysiology. This paper discusses an upper rate limiting algorithm in which the pacemaker leaves an atrial synchronized pacing modality when the atrial rate exceeds the upper rate limit and begins to pace in a rate decrementing VDI mode. This VDI mode is similar to the VVI mode, but atrial activity is monitored as well as ventricular activity.
Also important to an understanding of the present invention is an awareness of the rapidly advancing rate responsive pacing modalities.
Implantable sensors and transducers which permit estimation of a body's demand for oxygenated blood have become available and have been incorporated into pacemakers to control the pacing rate. Many variables have been monitored to estimate the patient's metabolic demand. One such pacer which monitors patient activity is shown by U.S. Pat. No. 4,428,378 to Anderson and Brumwell. Another pacer which monitors the patient's minute ventilation is shown in U.S. Pat. No. 4,596,251 to Plicchi and Canducci.
The atrial depolarization rate of the heart, however, is still regarded by many investigators as the best indicator for setting a pacing rate, and as a consequence, many investigators have attempted to combine an atrial synchronized or P-wave sensing pacemaker with sensor or transducer based pacing rate. One such effort has been disclosed in U.S. Pat. application ser. no. 125,422 filed Nov. 25, 1987. This DDD device has essentially two escape interval timers. The first timer is a V-A timer with a value selected by the physician to set a lower rate limit. The second timer is an activity modulated V-A timer reflecting the patient's activity level.
In operation, such a device paces at the first to expire V-A time. This operational sequence provides sensor based atrial pacing above a physician selected rate floor.
However, consider the case in which the patient is experiencing a pathologically high atrial rate. The atrial tracking behavior causes the ventricular rate to accelerate to the ventricular rate pacing limit while ignoring the sensor output which may indicate a more physiologic rate.
Such devices ignore the sensor output at high atrial rate and Wenckebach.